A JFET transistor is a known electronic switch that comprises a control gate whose function is to allow or disallow a current to flow between a drain and a source. Such a transistor is of the normally ON type if the voltage VDS between the Drain and the source is zero when the voltage VGS between the gate and the source is zero. This implies that the drain-source path is conducting or ON in the absence of a control voltage VGS between gate and source. Conversely, a JFET transistor is of the normally OFF type if the drain-source path is not conducting in the absence of a control voltage VGS between gate and source.
It turns out that a JFET electronic switch of the normally ON type offers much better performance than other types of electronic switches, such as MOSFETs, IGBTs or even JFETs of the normally OFF type. The reason for this is that such a switch offers, notably, the advantages of higher switching speed, lower losses in the conducting state, a better temperature stability, a smaller size and a lower cost.
Nevertheless, any electronic switch of the normally ON type has the drawback of being in the conducting state (or ON) in the absence of a control voltage on its gate. This feature is not fail-safe for the control of large currents since this switch allows the current to flow between drain and source in the absence of control on the gate. This clearly leads to high potential risks for the safety of equipment and people.
Usually, a speed controller of the frequency converter type comprises a rectifier stage responsible for rectifying a voltage coming from an external source of AC electrical power (for example a 380 VAC three-phase electrical supply network) in order to supply a DC voltage onto a DC bus (for example of around 400 to 800 Vcc, or more, depending on the conditions of use). One or more bus capacitors of high capacitance are typically used in order to keep the voltage on the DC bus constant.
The speed controller next comprises an inverter stage responsible for controlling an electric motor with a voltage of variable amplitude and frequency, starting from this DC bus. For this purpose, the inverter stage has two electronic power switches per phase. Each switch is controlled by a control circuit powered by a power supply of the SMPS (Switched-Mode Power Supply) type for the speed controller.
Thus, if it is desired to use electronic switches of the normally ON type in the inverter stage of a speed controller, this may then present several risks:                On power-up of the controller, it must be ensured that the circuits for controlling the electronic switches are correctly supplied with power, in order to be able to open these switches before the voltage appears on the DC bus of the speed controller,        On power-down, it must be ensured that the electronic switches are kept open until the bus capacitor or capacitors are discharged, for example into balancing resistors, being aware that the discharging of these capacitors can sometimes last for up to several minutes,        If a fault occurs, it must also be ensured that the switches are kept open until the capacitor or capacitors of the DC bus are discharged. Such a fault can arise notably from a short-circuit in the control circuit, from a loss of power to the control circuit, etc.        
In order to avoid these risks, a solution already exists in which each JFET transistor of the normally ON type is configured in series with another auxiliary component of the normally OFF type, such as for example a CASCODE transistor. This solution however requires the use of two transistors in series for each power switch, which notably leads to an increase in the production costs and an increase in the losses by conduction.